Alkaline protease activity of a local soil isolate of Penicillium sp. for soy protein hydrolysis has been explored under solid substrate fermentation (SSF) conditions. The optimum assay parameters for alkaline protease activity of Penicillium sp. were: pH 9.0 (borate buffer, 0.1 M) and temperature, 45 degreesC. Pre-incubation of enzyme extract with Fe3+, Hg2+ and Cu2+ drastically inhibited the enzyme activity, whereas Ca2+, Mg2+ and Mn2+ marginally enhanced it. Penicillium sp. grown at 28 degreesC for 3 days on wheat bran supplemented with 25 mg soy protein g(-1) bran, moistened with M-9 salt solution (1 ml g(-1)), inoculated with 1 x 10(10) spores g(-1) and extracted with 0.1 M borate buffer was found to yield a high titre of alkaline protease. (C) 2003 Elsevier Ltd. All rights reserved.